1. Field of the Invention
The present invention relates in general to adaptive digital filters for analog inputs; and more particularly, to an adaptive digital filter for eliminating alternating current noise from a direct current analog input signal in the control system of a pressurized light water nuclear power system.
2. Description of the Related Art
Adaptive digital filters are commonly used to perform many types of filtering when the signal to be filtered varies over time. For example, U.S. Pat. No. 4,106,102 to Desblache discloses a self-adaptive digital filter for noise and phase jitter reduction which is used with sinusoidal input signals. The filter described in Desblache is a narrow bandwidth filter for accepting or rejecting the undesired signal(s). In Desblache, the center frequency of the narrow bandpass filter is adjusted to follow variations in the input signal by separating the filtered signal into quadrature components, generating mutually in-quadrature components from the quadrature components and then generating an error signal from the quadrature and mutually in-quadrature components.
An apparatus for removing "phase jitter" was disclosed in Desblache utilizing the above-described filter. A similar problem, that of determining accurate timing differences between pulse periodic signals used in a LORAN C navigation system, is described in U.S. Pat. No. 4,416,017 to Jasper et al. In Jasper et al., the frequency of interfering signal is determined using an autocorrelation function and a phase-locked loop is used to adjust the sampling of pulsed periodic signals generated by LORAN C transmitters.
A different technique for attenuating interference is disclosed in U.S. Pat. No. 4,320,535 to Brady et al. A microwave receiving station for satellite transmissions is disclosed in Brady et al. which includes an auxiliary antenna aligned to receive an interfering signal. The signal received by the auxiliary antenna is passed through a quadrature modulator to adjust its amplitude to match that of the interference signal received by the main antenna and to shift its phase so that the signal output by the quadrature modulator can be combined with the signal received by the main antenna in a hybrid circuit which cancels the interference signal received by the main antenna.
One of the most common sources of noise in electrical systems is generated by alternating current power supplies. The expected or nominal frequency of conventional alternating current power supplies is normally 60 cycles per second in North America and normally 50 cycles per second in Europe. However, the actual frequency may vary over a range as large as .+-.5%, for example when a diesel generator is used to supply power. Furthermore, the frequency may differ from location to location even within a single installation, such as in a pressurized light water nuclear power plant in which more than one diesel generator is installed. Therefore, input lines carrying direct current analog input signals from a first location to a second location may be subjected to "power line hum" of a different frequency than that which is present at the second location. Conventional filters for removing power line hum sense the frequency of their local power supply which may have a different frequency than that which is present on the input lines. Such filters can be ineffective and potentially result in a reduction in the quality of the input signals.
An alternative conventional filter utilizes analog components to filter out the alternating current noise signal from the direct current input signals. However, filters using analog components are relatively slow to respond to sudden changes in the level of the input signals. Thus, a short and sudden fluctuation in a measured condition may be undetected by control systems which use this type of conventional analog filter.